The invention relates to a burner for feeding solid and liquid or gaseous fuel into a burning zone of a kiln, for instance a rotary kiln.
Such burners are known for instance from the German patent specifications DE 2905746 and DE 3027587 and may comprise an outer burner tube in which is mounted a central channel ending in a spray nozzle for feeding burning liquid or gaseous fuel such as heavy fuel oil, waste products of solvents, lubricating oils, natural gas and the like and primary air into a burning zone of a kiln for the heat treatment such as sintering in same of the kiln products, an concentric channel or a channel system surrounding the central channel which concentric channel(s) feeds/feed combustion air as primary air into the burning zone and which may be provided with air nozzles for creating an air swirl in the burning zone and yet another concentric channel surrounding the primary air feeding channel(s) for feeding solid fuel into the burning zone. The necessary combustion air for a sufficient combustion in the burning zone of the kiln is provided partly by the primary air fed to the burning zone through the burner proper, cf. above, partly by secondary air such as spent cooler air from the kiln cooler fed directly to the burning zone. The primary air from such burners has to be fed to the burning zone at a high velocity rate as it is imperative for maintaining an appropriate size of the burning flame that the burner provides a considerable momentum feed of fuel and air per time unit.
The burner thus ejects jets of fuel and primary air and these jets have also to be powerful enough to be able to take in the total amount of secondary air into the burning zone and to form air/material recirculation zones in same ensuring the ignition of the fuel.
This momentum feed per time unit is a combination of the total of mass flows (kg/s) out of the burner multiplied by their respective outlet velocities (m/s). Usually the primary air has to be fed to the burning zone in a fairly cold state by fans or compressors ensuring a sufficiently high air velocity for the desired jet effect, because such fans or compressors might be damaged in case heated gases and especially in case heated, dust-laden gases were used as primary air. Further, preheating of the primary air has hitherto been avoided a.o. due to the risk of coking or preignition of the fuel before the latter arrives into the burning zone or risk of the burner construction loosing its mechanical strength through the heating and thereby being bent by its own weight. However, the use of cold, primary air causes an undesired heat loss in the kiln system as such and efforts have therefore been made to reduce the amount of primary air (kg/s) in favour of a corresponding increase in the amount of secondary, preheated air to be fed to the burning zone. This could be obtained increasing only the velocity of the primary air for taking in the secondary air thereby keeping the amount of fed primary air to a minimum, but would in return entail the use of more complicated and expensive and thus also more vulnerable and heavier fan equipment such as compressors instead of the normally preferred, simple centrifugal fans.